Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker may be considered to be comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, having electrodes which electrically couple the pacemaker to the heart. A lead may provide both unipolar and bipolar pacing and/or sensing electrode configurations. In the unipolar configuration, the pacing stimulation pulses are applied or intrinsic responses are sensed between a single electrode carried by the lead, in electrical contact with the desired heart chamber, and the pulse generator case. The electrode serves as the cathode (negative pole) and the case serves as the anode (positive pole). In the bipolar configuration, the pacing stimulation pulses are applied or intrinsic responses are sensed between a pair of closely spaced electrodes carried by the lead, in electrical contact with the desired heart chamber, with the most proximal electrode serving as the anode and the most distal electrode serving as the cathode.
Pacemakers deliver pacing pulses to the heart to induce a depolarization and a mechanical contraction of that chamber when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses in one chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode.
Recently, there has been the introduction of pacing systems that stimulate in corresponding chambers of the heart as, for example, the right ventricle (RV) and left ventricle (LV). These are termed biventricular stimulation devices.
Pacemaker Mediated Tachycardia (PMT) also called “endless-loop tachycardia”, or “pacemaker reentrant tachycardia”, is a recognized pacemaker related rhythm anomaly. PMT can result in any pacemaker capable of sensing and responding to atrial depolarizations when A-V synchrony is dissociated, typically by a premature ventricular complex (PVC). A PVC, as is known, is a native depolarization arising from an ectopic location in the ventricle and occurring early with respect to the next expected conducted ventricular depolarization. Basically, it is an R wave that is not preceded by an atrial event. Such ventricular events may conduct in a retrograde direction to the atria and cause atrial depolarizations. If this atrial depolarization occurs after completion of the PVARP, the device is able to sense this retrograde atrial depolarization, classify it as an alert event, and then, after the appropriate AV delay, delivers a stimulus to the ventricle. Thus, the device provides the anterograde conduction pathway for the reentrant circuit and the intrinsic conduction system of the heart provides the retrograde pathway. A repetitive cycle of ventricular pacing synchronized to the retrograde P-wave can ensue.
Premature ventricular complex (PVC) events are actually the most common trigger for a pacemaker mediated tachycardia (PMT). In the patient whose conduction system can support retrograde conduction, the other prerequisite is that the AV node and atrium be physiologically recovered and hence, not refractory. This occurs when there is AV dissociation. The most common setting for this is the presence of a PVC. In the art, several different methods and hence algorithms, such as extending PVARP, “A pace on PMT” and “A pace on PVC”, etc., have been implemented to deal with this inappropriate rhythm disorder. Each method has its unique limitations. In fact, still newer methods and algorithms have been designed to mitigate the limitations of the previous methods and algorithms.
For example, the PVARP extension may simply postpone development of a PMT. In addition, PVARP extensions predispose the heart to sustained loss of atrial tracking when there is intact conduction with a first degree AV block.
The “A pace on PMT” algorithm is intended to terminate a PMT that has already developed. It calls for the intentional delivery of an atrial pacing stimulus after retrograde P-waves are confirmed but at a time when the atrial myocardium should no longer be physiologically refractory. The atrial stimulus will capture the atrium and AV node in the anterograde direction blocking retrograde conduction from the ensuing ventricular paced complex and terminating the PMT.
The “A pace on PVC” algorithm, on the other hand, calls for the delivery of an atrial pacing pulse after a PVC event with the objective of preventing a PMT. An atrial pacing stimulus is delivered after a preset delay when a P-wave (presumed a retrograde P wave) is detected within the PVARP after the PVC event. The atrial stimulus will capture the atrium and AV node in the anterograde direction in order to prevent the PMT from happening. However, the “A pace on PVC” algorithm can create a long short sequence that may initiate another type of tachycardia known as a supraventricular tachycardia.
In one instance, for example, an R-wave was properly detected as a PVC. As a result, an atrial pacing pulse was provided 319 ms after the P-wave (presumed to be a retrograde P wave) according to the “A Pace on PVC” algorithm. While this prevented a PMT, it initiated a non-sustained atrial tachycardia at a cycle length of approximately 270 ms. While the system correctly identified the PVC event, this PVC event was not associated with a retrograde P wave but in fact, a sinus P wave. Had the P wave actually been a retrograde P wave, an undesired intrinsic tachycardia would not have resulted in this case.
Hence, there is a need in the art to improve the specificity of the A-pace on PVC response to insure that the P wave detected after a PVC event is a retrograde P wave. The invention addresses these and other issues.